The UNI-SOLAR^®-Triple-Junction-Technology: the only thin film photovoltaic technology that is really fit for mass-production.

1. The UNI-SOLAR^® Triple-Junction Technology.

In contrast with almost all other kinds of solar cells, the UNI-SOLAR^® thin film solar cells are flexible and unbreakable. These cells are produced using roll-to-roll vacuum deposition processes on a continuous roll of stainless steel. First a metal reflector is deposited, followed by three different thin film cells (p-i-n structure), one on top of the other; the band-gap of each i-layer is adapted to a specific part of the solar spectrum. See figure 1. This structure is covered with a transparent conductive electrode of Indium-Tin-Oxide (ITO). The cell directly under this electrode converts the blue light, the middle cell the yellow/green range of the solar spectrum and the bottom cell the red light. The coated rolls are then cut into solar cells ('Strip Cells') of different cell dimensions.

	Figure 1: Structure of a UNI-SOLAR®-Cell.

Depending on the dimensions of the cells used and the interconnection pattern used, different types of modules originate. The modules are encapsulated into polymers by vacuum lamination and using EVA and TEFZEL^®, a fluoro-polymer-film from DuPont. The materials used for the back-side of the modules also depend on the product type.

Because each UNI-SOLAR^®-cell basically consists of three cells on top of each other, the nominal Strip Cell voltage is already 1.5 V. Hence, three times less interconnections (compared to c-Si cells) are required to build up a certain PV-module voltage, which gives rise to cost reductions. At present, the production capacity of UNI-SOLAR^®- products is increased from 5 MW to 30 MW with an additional 25 MW-production line. This will lead to considerable sales price reductions in the next three years.

2. About United Solar Ovonic

United Solar Ovonic, building on technology invented and pioneered by ECD Ovonics, is the world leader in thin-film amorphous photovoltaics. Its existing 25-megawatt production equipment is the world’s largest and most advanced machine for the manufacture of thin-film amorphous silicon alloy solar cells and related products used for a variety of applications ranging from large solar farms for utility-scale applications to charging batteries for solar lanterns. UNI-SOLAR^® solar cells are lightweight, rugged and flexible, and are ideal as building-integrated photovoltaic roofing systems for residential and industrial customers. ECD Ovonics and United Solar Ovonic hold the basic patents covering the continuous roll-to-roll manufacturing of thin-film amorphous silicon alloy multi-junction solar cells and related products. More information on United Solar Ovonic can be found at www.uni-solar.com.

3. UNI-SOLAR^® Products.

The above-described technology is enormously versatile, leading to a large product range. Of course, Bekaert ECD Solar Systems LLC produces rigid framed modules (UNI-SOLAR^® US-Series from 3 Wp up to 64 Wp), but also unique flexible modules (UNI-SOLAR^® USF-Series from (5 Wp up to 32 Wp) and even foldable flexible modules (UNI-SOLAR^® UNI-PACTM-Series, 10.5 Wp up to 30 Wp). These flexible finished products (see figure 2) have a poly-urethane foam material and a vinyl-foil at the backside. The foldable modules are hand-finished with a coated nylon.
 

Fig 3 UNI-SOLAR®-products for building integration

The real potential of the UNI-SOLAR^®-technology is however only fully exploited in the photovoltaic products for building integration (BIPV): those products ARE the roof, which leads to a considerable reduction in cost of ownership. Finished products' range consists of PV-standing seam roofing elements (UNI-SOLAR^® ASR and SSR, 128 Wp or 64 Wp elements) or PV-shingles (UNI-SOLAR^® SHR-17, 17 Wp), (Fig. 3). In addition, Bekaert ECD Solar Systems LLC produces half-products such as PV-laminates which can be used by OEM-customers in the building sector for integration with metal panels (e.g. Galvalume^®-steel standing seam metal roofing elements or façade elements) or membrane roofing materials. The UNI-SOLAR^® PVL-64 and PVL-128, which have a peel-and-stick cold bonding foil at the back-side, belong to this category of products.

4. Performance and effective energy generation of UNI-SOLAR^®-products.

The stabilised aperture efficiency of UNI-SOLAR^®-modules is 7.0 %. Therefore 1.7 times the surface-area from poly-crystalline modules is needed to build a 1 kWp-system. But in most cases, this is not really a disadvantage. The installed cost per kWp of a building integrated PV-system with UNI-SOLAR^® modules is comparable with the cost of a poly-crystalline system. In a private house (footprint 12 X 10 meter with a saddle roof, inclination 45°) the south-oriented part of the roof has more than 80 m2 of available surface area. So there is enough space to install about 5.1 kWp with UNI-SOLAR^®-technology. Remark that over 90 % of the systems installed on private homes have a capacity of only 2 - 4 kWp. So, there is more than enough space available in most cases to work with the UNI-SOLAR^®-technology.

UNI-SOLAR^®-PV-systems generate considerably more (10 - 15 %) electricity (in kWh) per kWp and per year than systems with competing products. In Western- and Southern-Europe, an AC-output between 800 and 1100 kWh/kWp per year can be expected, depending on the exact location and quality of BOS-components. The UNI-SOLAR^®-systems have therefore a shorter payback time. This is due to the following intrinsic properties of the UNI-SOLAR^®-technology: a higher conversion efficiency under low light conditions (the efficiency actually goes up from 7.0 % at 1000 W/m2 to 7.6 - 7.7 % at 100 W/m2) and a much better use of the diffuse light (e.g. when the sky is covered with clouds). The UNI-SOLAR^®-products are further extremely shadow tolerant: over each cell, there is a by-pass-diode, so that only the output of the shaded cell is (partly) lost. And finally, the power output of UNI-SOLAR^®-products is almost independent on ambient temperature (between 13°C and 30°C).

In this context we refer to the publication of Photon International in November 2000 ("In Holland, the sun prefers amorphous modules", p. 10-11).

N.V. Bekaert ECD Solar Systems Europe (BESS EUROPE),
Karreweg 13, B-9870 Zulte, Belgium.
Tel. +32-(0)9-338 59 25; Fax: +32-(0)9-338 59 11
e-mail: jenny.dierick@bekaert.com
web site: www.uni-solar.com

 

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